Time-division multiplexing (TDM) is a type of digital multiplexing in which pulses representing bits from different channels are interleaved in time. In telecommunications, T-carrier is the generic designator for any of several TDM telecommunications carrier systems in North America and Japan, while E-carrier system, where ‘E’ stands for European, is compatible with the T-carrier and is used just about everywhere else in the world.
The designators for T-carrier in digital hierarchy correspond to the designators for digital signal (DS) level hierarchy. For example, a DS-1 is the data carried on a T-1 carrier, and a DS-3 signal is carried on a T3 carrier. The basic unit of the T-carrier system is the DS-0, which has a transmission rate of 64 kbit/s and is commonly used for one voice circuit. A T1 line refers to a combination of data circuits and operates at a 1.544 Mbit/s line rate. The T1 carrier carries 24 pulse-code-modulated, time-division-multiplexed speech channels each encoded in 64 kbit/s streams, leaving 8 kbit/s of framing information which facilitates the synchronization and demultiplexing at the receiver. Similarly, the T3 carries 672 voice channels or 24 DS-1 channels.
Integrated services digital network (ISDN) is an international telecommunications standard for providing a digital service from the customer's premises to the dial-up telephone network. ISDN uses 64 Kbps circuit-switched channels, called “B channels” (bearer channels), to carry voice and data and a separate D channel (delta channel) for control signals. The D channel signals the carrier's voice switch to make calls, put them on hold and activate features, such as conference calling and call forwarding. It also receives information about incoming calls, such as the identity of the caller. Since the D channel connects directly to the telephone system's SS7 signaling network, ISDN calls are connected much faster than regular telephone calls.
ISDN's basic service is basic rate interface (BRI), which is made up of two 64 Kbps B channels and one 16 Kbps D channel (2B+D). If both channels are combined into one, called “binding,” the total data rate becomes 128 Kbps and is four and a half times the bandwidth of a V.34 modem (28.8 Kbps). ISDN's high-speed service is primary rate interface (PRI). It provides 23 B channels and one 64 Kbps D channel (23B+D), which is equivalent to the 24 channels of a T1 line. Alternatively, channel associated signaling (CAS) communications may be used in which signaling information is carried within the data channels of a T1 line (in band) rather than on a separate control (D) channel. When several channels are bonded together, high data rates can be achieved. For example, it is common to bond six channels for quality videoconferencing at 384 Kbps.
DS-1 signals are frequently used to connect equipment within a facility. When a DS-1 leaves the building, it becomes a T1 and is referred to as a span. The signal is boosted to a higher level and superimposed on a DC voltage, enabling repeaters in the field to be powered from the span itself. Repeaters are placed every few thousand feet, to clean up and strengthen the signal. DS-3 signals are almost exclusively used within buildings because a T-3 carrier can only go about 600 feet between repeaters. The T-carrier system traditionally uses in-band signaling or bit robbing, resulting in lower transmission rates than the E-carrier system, which lowers the effective data rate to 56 Kbps over a nominal 64 Kbps channel.
As can be appreciated, the use of dedicated T1 (or T2 or T3) lines can be quite costly due to the distance limitations, repeaters required, and the necessity to dedicate specific conductors for exclusive use. In contrast, packet networks are more readily available and less costly to use. It is therefore desirable to carry TDM traffic over a packet network to avoid the costs associated with carrying conventional TDM traffic over a T1 line.
Some solutions have been proposed to carry TDM traffic over a packet network, such as ATM Forum's Circuit Emulation Service (CES) Interoperability Specification, version 2.0, January, 1997. The CES approach, however, is limited to the situation where two separate TDM networks are inter-connected by an asynchronous transfer mode (ATM) network, which means the packet network is only relaying the TDM traffic and doesn't actually terminate the TDM traffic. That is, the packet network can't provide TDM related services, and so another TDM network is required. Accordingly, the CES model doesn't work in situations where TDM traffic is sent to a node within an ATM network for termination within the ATM network.
In another solution proposed by IETF, pseudo-wire emulation edge-to-edge (PWE3) employs the same approach as CES, but is not restricted to ATM networks. Internet protocol (IP), multiprotocol label switching (MPLS), FrameRelay, and ATM networks are all supported.
Another solution that allows connection of TDM services in an ATM network is ATM Forum's Voice and Multimedia Over ATM Loop Emulation Service Using AAL2 (LES) Specification, July, 2000. In this solution, the ATM network terminates the TDM service, but it is done per local loop line, i.e., per DS-0. It doesn't support the service on a T1/E1 level, T3/E3 level. Accordingly, it does not act as a T1 line, for example, in all respects.
What is needed is a method for providing TDM terminating service over a packet network that terminates the TDM traffic in the packet network, instead of relaying it to another TDM network, so that TDM terminating services may be provided in the packet network.